In cereals, starch makes up approximately 45-65% of the weight of the mature grain. The starch is composed of two types of molecule, amylose and amylopectin. Amylose is an essentially linear molecule composed of α-1,4 linked glucosidic chains, while amylopectin is highly branched with α-1,6 glucosidic bonds linking linear chains.
The synthesis of starch in the endosperm of higher plants is carried out by a suite of enzymes that catalyse four key steps. Firstly, ADP-glucose pyrophosphorylase activates the monomer precursor of starch through the synthesis of ADP-glucose from G-1-P and ATP. Secondly, the activated glucosyl donor, ADP-glucose, is transferred to the non-reducing end of a pre-existing α1-4 linkage by starch synthases. Thirdly, starch branching enzymes introduce branch points through the cleavage of a region of α-1,4 linked glucan followed by transfer of the cleaved chain to an acceptor chain, forming a new α-1,6 linkage. Starch branching enzymes are the only enzymes that can introduce the α-1,6 linkages into α-polyglucans and therefore play an essential role in the formation of amylopectin. Finally, starch debranching enzymes remove some of the branch linkages although the mechanism through which they act is unresolved (Myers et al., 2000).
While it is clear that at least these four activities are required for normal starch granule synthesis in higher plants, multiple isoforms of each of the four activities are found in the endosperm of higher plants and specific roles have been proposed for individual isoforms on the basis of mutational analysis (Wang et al, 1998, Buleon et al., 1998) or through the modification of gene expression levels using transgenic approaches (Abel et al., 1996, Jobling et al., 1999, Scwall et al., 2000). However, the precise contributions of each isoform of each activity to starch biosynthesis are still not known, and it is not known whether these contributions differ markedly between species. In the cereal endosperm, two isoforms of ADP-glucose pyrophosphorylase are present, one form within the amyloplast, and one form in the cytoplasm (Denyer et al., 1996, Thorbjornsen et al., 1996). Each form is composed of two subunit types. The shrunken (sh2) and brittle (bt2) mutants in maize represent lesions in large and small subunits respectively (Girouz and Hannah, 1994). Four classes of starch synthase are found in the cereal endosperm, an isoform exclusively localised within the starch granule, granule-bound starch synthase (GBSS), two forms that are partitioned between the granule and the soluble fraction (SSI, Li et al., 1999a, SSII, Li et al., 1999b) and a fourth form that is entirely located in the soluble fraction, SSIII (Cao et al, 2000, Li et al., 1999b, Li et al, 2000). GBSS has been shown to be essential for amylose synthesis (Shure et al., 1983), and mutations in SSII and SSIII have been shown to alter amylopectin structure (Gao et al, 1998, Craig et al., 1998). No mutations defining a role for SSI activity have been described.
Three forms of branching enzyme are expressed in the cereal endosperm, branching enzyme I (SBEI), branching enzyme IIa (SBEIIa) and branching enzyme IIb (SBEIIb) (Hedman and Boyer, 1982, Boyer and Preiss, 1978, Mizuno et al., 1992, Sun et al., 1997). In maize and rice, high amylose phenotypes have been shown to result from lesions in the SBEIIb gene, also known as the amylose extender (ae) gene (Boyer and Preiss, 1981, Mizuno et al., 1993; Nishi et al., 2001). In these SBEIIB mutants, endosperm starch grains showed an abnormal morphology, amylose content was significantly elevated, the branch frequency of the residual amylopectin was reduced and the proportion of short chains (<DP17, especially DP8-12) was lower. Moreover, the gelatinisation temperature of the starch was increased. In addition, there was a significant pool of material that was defined as “intermediate” between amylose and amylopectin (Boyer et al., 1980, Takeda, et al., 1993b). In contrast, maize plants mutant in the SBEIIa gene due a mutator (Mu) insertional element and consequently lacking in SBEIIa protein expression were indistinguishable from wild-type plants in the branching of endosperm starch (Blauth et al., 2001), although they were altered in leaf starch. Similarly, rice plants deficient in SBEIIa activity exhibited no significant change in the amylopectin chain profile in endosperm (Nakamura. 2002).
In maize, the dull1 mutation causes decreased starch content and increased amylose levels in endosperm, with the extent of the change depended on the genetic background, and increased degree of branching in the remaining amylopectin (Shannon and Garwood, 1984). The gene corresponding to the mutation was identified and isolated by a transposon-tagging strategy using the transposon mutator (Mu) and shown to encode the enzyme designated starch synthase II (SSII) (Gao et al., 1998). The enzyme is now recognized as a member of the SSIII family in cereals. Mutant endosperm had reduced levels of SBEIIa activity associated with the dull1 mutation. No corresponding mutation has been reported in other cereals. It is not known if these findings are relevant to other cereals, for example barley.
WO94/09144 suggests the use of sense and antisense genes to alter the natural ratios of starch synthase (SS) and SBE in maize. However, no data are presented to substantiate the proposed molecular strategies and there is no suggestion of specifically reducing the activity of SBEIIa.
In potato, down regulation of SBEI alone causes minimal affects on starch structure (Filpse et al., 1996), although further work identified some qualitative changes (Safford et al., 1998). However, in potato the down regulation of SBEII and SBEI in combination increased the relative amylose content much more than the down-regulation of SBEII alone (Schwall et al., 2000).
Two types of debranching enzymes are present in higher plants and are defined on the basis of their substrate specificities, isoamylase type debranching enzymes, and pullulanase type debranching enzymes (Myers et al., 2000). Sugary-1 mutations in maize and rice are associated with deficiency of both debranching enzymes (James et al., 1995, Kubo et al., 1999) however the causal mutation maps to the same location as the isoamylase-type debranching enzyme gene. In the Chlamydomonas sta-7 mutant (Mouille et al., 1996), the analog of the maize sugary-1 mutation, isoamylase activity alone is down regulated. Starch biosynthesis genes that have been cloned from cereals are listed in Table 1.
Starch is widely used in the food, paper and chemical industries. The physical structure of starch can have an important impact on the nutritional and handling properties of starch for food or non-food or industrial products. Certain characteristics can be taken as an indication of starch structure including the distribution of amylopectin chain length, the degree of crystallinity and the presence of forms of crystallinity such as the V-complex form of starch crystallinity. Amylopectin chain length may be an indicator of altered crystallinity and altered gelatinisation and is also thought to have a correlation with reduced retrogradation of amylopectin. Additionally, varied amylopectin chain length distribution is thought to reflect organoleptic properties of food in which the starch is included in significant amounts. Reduced crystallinity of a starch may also be indicative of a reduced gelatinisation temperature of starch and is thought to be associated with enhanced organoleptic properties.
The relatively high gelatinisation temperature of most high amylose starches is a disadvantage for certain food applications. Gelatinisation temperature is reflective of the comminution energy required to process such foods. Higher temperatures are normally required to process grain or flour to manufacture foods from such grains or starches. Therefore, products having high amylose starches are generally more expensive. In addition, consumers may need to use longer times and higher temperatures to prepare the manufactured foods or to make foods from flour having high amylose starches. High amylose starches having reduced or normal gelatinisation temperatures would be advantageous in many food applications.
Starch composition, in particular the form called resistant starch, has important implications for bowel health, in particular health of the large bowel. Accordingly, high amylose starches have been developed in certain grains such as maize for use in foods as a means of promoting bowel health. The beneficial effects of resistant starch result from the provision of a nutrient to the large bowel wherein the intestinal microflora are given an energy source which is fermented to form inter alia short chain fatty acids. These short chain fatty acids provide nutrients for the colonocytes, enhance the uptake of certain nutrients across the large bowel and promote physiological activity of the colon. Generally if resistant starches or other dietary fibre is not provided the colon is metabolically relatively inactive.
Another nutritional component of the grains and in particular of barley is β-glucan. β-glucan consists of glucose units bonded by β (1-4) and/or β (1-3) glycosidic linkages and are not degraded by human digestive enzymes, making them suitable as a source of dietary fibre. β-glucans can be partially digested by endogenous colonic bacteria which fermentation process gives rise to short chain fatty acids (predominantly acetate, propionate and butyrate) which are beneficial to mucosal cells lining the intestine and colon (Sakata and Engelhard, 1983). Ingestion of β-glucan also has the effect of increasing bile acid excretion leading to a reduction in total serum cholesterol and low density lipoproteins (LDL) with a lowering of the risk of coronary disease. Similarly β-glucan acts by attenuating excursions in postprandial blood glucose concentration. It is thought that these effects may also be based on the increase of viscosity in the contents of the stomach and intestines.
Whilst modified starches or β glucans, for example, can be utilised in foods that provide functionality not normally afforded by unmodified sources, such processing has a tendency to either alter other components of value or carry the perception of being undesirable due to processes involved in modification. Therefore it is preferable to provide sources of constituents that can be used in unmodified form in foods.
Barley (Hordeum vulgare) is the fourth largest cereal grain crop produced worldwide and is relatively underutilized in terms of human consumption aside from its use to produce alcoholic beverage. On average, barley grain contains about 64% starch, 11% protein and 5% β-glucan (normally 3-6%). The remaining 20% includes moisture, fiber and other minor components.
Known variation in barley starch structure is limited relative to the variation available in maize. Mutants in SBEIIb, corresponding to the amylose-extender phenotypes in maize or rice, have not been characterized in barley. The phenotype conferred by SBEIIa or SBEIIb mutations in barley is unknown. The most highly characterised mutations are waxy and a high amylose mutation identified as AC38. High Amylose Glacier (AC38) has relatively modest increases in amylose content to a maximum of about 45% of total starch. Double mutants with a waxy phenotype have also been constructed and analysed (Schondelmaier et al., 1992; Fujita et al, 1999).
Other mutants of barley having high amylose starch contents have been identified. Chemically induced mutants in the SSIIa gene had higher levels of amylose in kernel starch, to about 65-70% (WO 02/37955 A1). The mutants M292 and M342 also showed substantially reduced average grain weight as a consequence of reduced starch synthesis, from a mean weight of about 51 mg for the parent line Himalaya to 32 and 35 mg for M292 and M342, respectively. Although the mutants retained the length and width of the wild-type grain, they were flattened from 2.8 mm average thickness for Himalaya to 1.6-1.8 mm thickness and had an essentially unfilled central region, which resulted in poorer milling characteristics. The ratio of grain length (L) to thickness (T) was found to be a useful diagnostic parameter for the mutant alleles, with mutants and wild-type seeds having an L:T ratio of >3.5 and <3.5 respectively. The starch content of the mutant lines was reduced from 49.0% for Himalaya to 17.7 and 21.9% for M292 and M342, respectively. It was shown that while there was a decrease in amylose content per grain from 6.2 mg per caryopsis to 4.0 and 4.8 mg in M292 and M342, respectively, there was a dramatic reduction in amylopectin content per caryopsis from 18.7 in Himalaya to 1.6 and 2.9 mg in the mutants. This showed that the high relative amylose level was a result of decreased amylopectin production. Grain β-glucan levels were increased in the mutants to above 10%. The starch showed reduced gelatinisation temperatures. The SSIIa mutants had an altered distribution of SBEIIa and SBEIIb activities between the starch granule and soluble fractions of the endosperm, however, they were essentially unaltered in the level of these activities in the endosperm as a whole (WO 02/37955; Morell et al., 2003).
Whilst elevated amylose starches of these types are useful, a barley starch with higher amylose contents is preferred, in particular if associated with improved starch synthesis and other characteristics, for example a reduced need for post-harvest modification. Such starch products are also relatively resistant to digestion and bring a greater health benefit.
General
Those skilled in the art will be aware that the invention described herein is subject to variations and modifications other than those specifically described. It is to be understood that the invention described herein includes all such variations and modifications. The invention also includes all such steps, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any two or more of said steps or features.
Throughout this specification, unless the context requires otherwise the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. The present invention is not to be limited in scope by the specific embodiments described herein, which are intended for the purposes of exemplification only. Functionally-equivalent products, compositions and methods are clearly within the scope of the invention, as described herein.
Bibliographic details of the publications referred to by author in this specification are collected at the end of the description. The references mentioned herein are hereby incorporated by reference in their entirety. Reference herein to prior art, including any one or more prior art documents, is not to be taken as an acknowledgment, or suggestion, that said prior art is common general knowledge in Australia or forms a part of the common general knowledge in Australia.
As used herein, the term “derived from” shall be taken to indicate that a particular integer or group of integers has originated from the species specified, but has not necessarily been obtained directly from the specified source.
The designation of nucleotide residues referred to herein are those recommended by the IUPAC-IUB Biochemical Nomenclature Commission, wherein A represents Adenine, C represents Cytosine, G represents Guanine, T represents Thymidine.